Aims. We attempt to establish the real nature of the orbital period variation and its relation to the spot activity of V711 Tau, and determine why the (B − V) colour of the star appears to be nearly independent of its V magnitude. We wish to verify whether existing predictions in the literature for the long-term spot activity of the star are true or not by making extended photometric observations, and whether the broad component of Hα emission originates in more localised active regions, as suggested by some chromospheric models, by searching for any correlation between the base-line width of the emission and the light modulation. Methods. We obtained new radial velocities of the G-type component of V711 Tau on 42 nights during 2004−09, and BV photometry of V711 Tau on 202 nights during the years 1993−2009. We measured the equivalent widths of two well resolved lines of the G-type component from 13 of the spectra obtained by us. We also measured the equivalent widths and base-line widths of Hα emission from 21 spectra obtained during 2008−09. We analyse these along with relevant information available in the literature. Results. The available radial velocity data of V 711 Tau are consistent with a sinusoidal modulation of its orbital period; the period of modulation is found to be 36.3 ± 1.9 yr. The fractional light-loss over a photometric cycle is found to vary with an average period of 14.1 ± 0.3 yr. It appears that physical processes linked to the magnetic activity of the spotted star are responsible for the variation in the orbital period. The excess reduction in the B band flux relative to that in V band caused by spot activity is almost compensated for by the fractional increase in the contribution by the hotter companion to the total light in the blue spectral region, and thereby makes the (B − V) colour of the binary system nearly independent of its V magnitude. We find that the ratio of the radii of the components derived from their v sin i values, which is usually quoted in the literature, is substantially larger than that implied by the ratio of their brightnesses in V band. There is a slight indication from the V band data that the spot activity in V711 Tau has a component that is fixed in the orbital frame of reference. The equivalent width and base-line width of Hα emission do not show any obvious correlation with the V band light curve.

Aims. The RS CVn star UX Ari appears bluer when it is fainter, which is unusual for a spotted star; we aim to resolve this unusual behaviour of the object. Also the data available in the literature indicate that the mean V magnitude of UX Ari varies by about 25 years; we want to confirm whether this is indeed true. Methods. The most important parameter in understanding the colour variation is the relative brightnesses of the component stars of UX Ari. We use all spectroscopic information available in the literature to obtain a reasonable value for this parameter. We also present new BV photometry of UX Ari obtained on 58 nights during 2001-07. Results. The V magnitudes of UX Ari we obtained are significantly fainter than those reported in the literature, and they are not consistent with a 25-year period for the mean light level. The increased fractional contribution by the hotter G-type component to the total light in the blue spectral region as the active star becomes faint, seems to be the real cause for the bluer colour of UX Ari at fainter visual magnitudes.

We present extensive UBVRI photometry of the active RS CVn binary DM UMa obtained at two different observatories over 1988–2008. We find the light curve of the star to be highly variable. The long‐term photometry shows no evidence of any cyclic spot activity. On one occasion, the amplitude of modulation in V band was 0.30 mag, a rather high value for an active star in a binary seen at a comparatively low orbital inclination. The mean V magnitude of DM UMa during 1993–94 was brighter than that over 1979–85 by about 0.35 mag. We interpret the monotonic increase in mean brightness from 1984 onwards as due to the steady disappearance of star‐spots in the near‐polar latitudes of the star. The V‐band data clearly show evidence for the presence of two distinct regions of enhanced spot activity on the visible primary, which are fixed in the orbital frame of reference, one facing the companion star and the other away from it. It appears that the nearby companion star suppresses the natural tendency of the active star for differential rotation and modifies the physical processes leading to the formation of spots, besides spinning it up by locking its rotation with the orbital motion. The variations in (V−R) and (V−I) colours with the V magnitudes clearly indicate that star‐spots about 800 K cooler than the unspotted photosphere are responsible for the rotational modulation of light in DM UMa. The spot‐filling factors derived from TiO‐band strengths, which are available in the literature, are found to show the expected anticorrelation with the V magnitudes of the star. Apparently, there is excess flux in U and B bands, which increases as the star becomes fainter, partly compensating for the reduction of flux in those bands due to spot activity. The excess flux, probably, originates from plages or facular regions associated with the spot activity, and indicates that the simple two‐component spot model with spotted and unspotted photospheric regions is not adequate to represent the star‐spot activity in DM UMa.

A search for rapid variability of Ha with a time resolution from 30 to 45 seconds is performed for c|) Per. In total, 42 spectra in Ha were obtained during three nights. Our results show the presence of rapid irregular variability (time scale of a few minutes) of the equivalent width of the Ha line in 4> Per.

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